The present invention relates to an image forming apparatus provided with a toner image holding belt, such as a photosensitive belt into which an electrostatic latent image to be developed into a toner image is written or an intermediate transfer belt onto which a toner image is primarily transferred from a toner image holding member.
The conventional image forming apparatus suffers from such problems as paper jam and image disarray, which arise from the fact that after passing an image transfer region, a recording medium, e.g., recording sheet, is not separated from an intermediate transfer belt and hence not transported to a fixing stage.
For the cause of the sheet clinging problem, it may be estimated that the recording sheet charged through the image transfer process is attracted to the toner image holding belt under a transfer electric field.
The following techniques (J01) and (J02), so far as we know, may be enumerated for the techniques for solving the poor-separation problem of the recording sheet having received a toner image from the toner image holding belt.
(J01) Technique disclosed in the Unexamined Japanese Patent Application Publication No. Hei 6-149074.
A sheet charge-removal member is located downstream of an image transfer stage for transferring a toner image onto the recording sheet, viz., between the recording-sheet holding the toner image and the photosensitive belt.
(J02) Technique disclosed in the Unexamined Japanese Patent Application Publication No. Hei 8-23719.
A separation plate consisting of an earth metal plate and an insulation shield plate, located downstream of a transfer stage for transferring a toner image to the recording sheet and on the sheet exit side of the transfer roller. The insulation shield plate is brought into contact with the earth metal plate from the transfer roller side and projected toward the transfer stage.
Those techniques have the following problems.
1) Problems of (J01)
The (J01) technique prevents the recording sheet from being electrostatically attracted to the photosensitive belt, by removing the charge from the recording sheet. The charge removal member is placed in proximity to the photosensitive belt and performs a discharging operation. The discharging operation adversely affects the electric characteristics of the photosensitive belt, and the products by the discharging operation attach to the photosensitive belt.
2) Problems of (J02)
The (J02) technique pushes the recording sheet in the direction in which the sheet moves apart from the toner holding belt by the utilization of the charge of the recording sheet per se. In case where the recording sheet used is a paper, not rigid, e.g., a thin paper containing water at high percentage, the separated sheet hangs down, so that the tip of the sheet comes in contact with the separation plate, resulting in paper jam and image disarray.
Accordingly, the present invention has the following objects:
(O01) to stably separate the recording sheet from the toner image holding belt, not using the charge remover requiring a high tension power source; PA1 (O02) to stably separate the recording sheet from the toner image holding belt, not locating a member that possibly obstructs the transportation of the recording sheet on the sheet transporting path; and PA1 (O03) to stably separate the recording sheet from toner image holding belt independently of ambient conditions and sheet characteristics. PA1 developing a toner image on a photosensitive member; PA1 forming a toner image on a toner image holding belt rotatably supported by a plural number of rolls including an inside transfer roll; PA1 moving a recording sheet through a transfer region when a toner image on said toner image holding belt passes through said transfer region; PA1 transferring the toner image from said toner image holding belt onto the recording sheet passing through said transfer region when voltage is applied to between said inside transfer roll and an outside transfer member, which is disposed in opposition to said inside transfer roll with respect to said toner image holding belt; and PA1 weakening an electric field developed between said outside transfer member and said inside transfer roller in a region located downstream of said transfer region, after the recording sheet passes through said transfer region. PA1 (A01) a toner image holding belt rotatably supported by a plural number of rolls (25 to 29) including an inside transfer roll (29), a toner image being formed on the surface of the toner image holding belt (B); PA1 (A02) an outside transfer member (30), which is disposed in opposition to the inside transfer member (29) with respect to the toner image holding belt (B) develops a transfer region (Q4) in a space between the outside transfer member (30) and the toner image holding belt (B); PA1 (A03) sheet transport means (42 to 47) for moving a recording sheet through the transfer region (Q4) when a toner image on the toner image holding belt (B) passes through the transfer region (B); PA1 (A04) a transfer unit (T2) including the inside transfer member (29), the outside transfer member (30) transfers the toner image from the toner image holding belt (B) onto the recording sheet that passes through the transfer region (Q4) when transfer voltage is applied to between the inside transfer member (29) and the outside transfer member (30); PA1 (A05) electric-field adjusting conductive member (51) disposed in proximity to and along the inner side of the toner image holding belt (B) leaving the transfer region (Q4) at a position located downstream of the transfer region (Q4); and PA1 (A06) electric-field adjust-potential applying means (53; 54) applies a potential, which weakens an electric field developed between the outside transfer member (30) and the inside transfer member (29) in a region located downstream of the transfer region (Q4), to the electric-field adjusting conductive member (51). PA1 (B01) a toner image holding belt rotatably supported by a plural number of rolls (25 to 29) including an inside transfer roll (29), a toner image being formed on the surface of the toner image holding belt (B); PA1 (B02) an outside transfer member (30), which is disposed in opposition to the inside transfer roll (29) with respect to the toner image holding belt (B) develops a transfer region (Q4) in a space between the outside transfer member (30) and the toner image holding belt (B); PA1 (B03) sheet transport means (42 to 47) for moving a recording sheet through the transfer region (Q4) when a toner image on the toner image holding belt (B) passes through the transfer region (B); PA1 (B04) a transfer unit (T2) including the inside transfer roll (29), the outside transfer member (30) transfers the toner image from the toner image holding belt (B) onto the recording sheet that passes through the transfer region (Q4) when transfer voltage is applied to between the inside transfer roll (29) and the outside transfer member (30); PA1 (B05) electric-field adjusting conductive member (55) for adjusting an electric field present upstream of the transfer region (Q4), the electric-field-adjust conductive member (55) being disposed in proximity to and along the inner side of the toner image holding belt (B) just before the transfer region (Q4); and PA1 (B06) electric-field adjust-potential applying means (57) applies a potential, which weakens an electric field developed between the outside transfer member (30) and the inside transfer roll (29) in a region located upstream of the transfer region (Q4), to the electric-field adjusting conductive member (55). PA1 (C01) a toner image holding belt (B) rotatably supported by a plural number of rolls (25 to 29) including an inside transfer roll (29), a toner image being formed on the surface of the toner image holding belt (B); PA1 (C02) an outside transfer member (30), which is disposed in opposition to the inside transfer roll (29) with respect to the toner image holding belt (B) develops a transfer region (Q4) in a space between the outside transfer member (30) and the toner image holding belt (B); PA1 (C04) a transfer unit (T2) including the inside transfer roll (29) and the outside transfer member (30) transfers the toner image from the toner image holding belt (B) onto the recording sheet that passes through the transfer region (Q4) when transfer voltage is applied to between the inside transfer roll (29) and the outside transfer member (30); PA1 (C05) elastic, electric-field adjusting conductive member (51) for adjusting an electric field present downstream of the transfer region (Q4), the electric-field-adjust conductive member (51) being disposed in contact with or in proximity to and along the inner side of the toner image holding belt (B) leaving the transfer region (Q4) at a position located downstream of the transfer region (Q4); and PA1 (C06) electric-field adjust-potential applying means (53) applies a potential, which weakens an electric field developed between the outside transfer member (30) and the inside transfer roll (29) in a region located downstream of the transfer region (Q4), to the electric-field adjusting conductive member (51). PA1 (A07) an insulation shield member (52) covering the end of the electric-field-adjust conductive member (51), which is closer to and faces the inside transfer roll (29). PA1 (B07) an insulation shield member (56) covering the end of the electric-field-adjust conductive member (55), which is closer to and faces the inside transfer roll (29). PA1 (C07) a rigid, plate-like insulating member (52), applied to the surface of the electric-field-adjust conductive member (51), which is opposite to its surface facing the image holding belt (B). PA1 (C08) the inside transfer roll (29), the outside transfer member (30) and the electric-field-adjust conductive member (51) are arranged so as to satisfy e.ltoreq.f, PA1 Pa: the upstream end of the electric-field-adjust conductive member (51), which is disposed in proximity to the inside transfer roll (29) PA1 Pb: intersection point where a straight line L1 coincident with the advancing direction of a recording sheet emanating from the transfer region (Q4) intersects a straight line L2 that passes through the upstream end Pa and is perpendicular to the straight line L1 PA1 Pc: intersection point where the straight line L2 intersects the image holding belt (B); PA1 e: distance between the upstream end Pa and the intersection point Pc PA1 f: distance between the intersection point Pb and the intersection point Pc. PA1 Pa: the upstream end of the electric-field-adjust conductive member (51), which is disposed in proximity to the inside transfer roll (29) PA1 Pb: intersection point where a straight line L1 coincident with the advancing direction of a recording sheet emanating from the transfer region (Q4) intersects a straight line L2 that passes through the upstream end Pa and is perpendicular to the straight line L1 PA1 Pc: intersection point where the straight line L2 intersects the image holding belt (B); PA1 e: distance between the upstream end Pa and the intersection point Pc PA1 f: distance between the intersection point Pb and the intersection point Pc. PA1 (C09) a downstream sheet guide (46), disposed downstream of the transfer region (Q4), for guiding a recording sheet leaving the transfer region (Q4), and the specific form 3 is defined as PA1 (C10) the inside transfer roll (29), the outside transfer member (30), the electric-field-adjust conductive member (51), and the downstream sheet guide (46) are arranged so as to satisfy h.ltoreq.g, PA1 P1: intersection point where the straight line L1 coincident with the advancing direction of the recording sheet emanating from the transfer region (Q4) intersects a straight line L3 which is prolonged from the downstream end of the electric-field-adjust conductive member (51), which is far away from the inside transfer roll (29), in the direction perpendicular to the straight line L1 PA1 P2: intersection point where the straight line L3 intersects the image holding belt PA1 g: distance between the intersection points P1 and P2. PA1 h: distance between the intersection points P1 and P3. PA1 P1: intersection point where the straight line L1 coincident with the advancing direction of the recording sheet emanating from the transfer region (Q4) intersects a straight line L3 which is prolonged from the downstream end of the electric-field-adjust conductive member (51), which is far away from the inside transfer roll (29), in the direction perpendicular to the straight line L1 PA1 P2: intersection point where the straight line L3 intersects the image holding belt PA1 g: distance between the intersection points P1 and P2 PA1 h: distance between the intersection points P1 and P3.
In the description of the "means to solve the problems", which is to be given below, reference symbols attached to portions are parenthesize for ease understanding of their correspondence to those in the "description of the preferred embodiments" to be given later.
It should be understood that the use of the parenthesize symbols does not limit the scope of the invention in any way.